The recently identified promoter of the anaerobically inducible operon of overlaps the Fnr (fumarate-nitrate reduction regulator)-dependent promoter. The Fnr-binding site of and the -35 hexamer sequence of are shared between the promoters. Inactivation of , through introduction of a -10 hexamer mutation, resulted in enhanced anaerobic induction of operon expression. The dependence on the ArcA (aerobic respiration control regulator) and Fnr transcription factors for anaerobic induction was tested for several and gene fusions. Anaerobic induction became more dependent on Fnr in derivatives lacking a functional promoter compared with wild-type constructs. Moreover, aerobic expression of the gene was reduced by the mutation, as was the dependence on ArcA for anaerobic induction. Inactivation of severely reduced Fnr-dependent anaerobic induction, in accord with previous findings. Transcription analyses demonstrated that a mutation in the -10 hexamer sequence of either or did not adversely affect transcription from the remaining promoter. Taken together, these results indicate that the promoter moderates the Fnr-dependent activation of through competition for RNA polymerase binding.


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